Digital television broadcast has newly started in addition to current analog television broadcast. The analog television broadcast and digital television broadcast are supposed to be transmitted simultaneously for a certain period of time. For this period, the digital television broadcast is transmitted with lower transmission power than that of the analog television broadcast, or is transmitted using a channel between analog television broadcast channels, in order not to affect the analog television broadcast. Therefore, digital broadcast receivers need sensitivity of reception in a weak electric field and properties of being able to endure interferences from neighboring channels.
Also in view of a cost and size of the product, a demand has become higher for an analog/digital-compatible front-end module capable of receiving both analog television broadcast and digital television broadcast having an analog demodulator and a digital demodulator provided in one receiving circuit in place of an analog receiving tuner and a digital receiving tuner provided separately.
Accordingly, various analog/digital-compatible front-end modules have been proposed (see Patent Documents 1-4).
Patent Document 1. JP 11-98426 A
Patent Document 2. JP 11-261913 A
Patent Document 3. JP 2000-59707 A
Patent Document 4. JP 2003-204274 A
For example, an analog/digital-compatible front-end module shown in FIG. 2 includes a tuner unit 17 for tuning-processing a high frequency signal received by an antenna 1, and a demodulation unit 21 for processing an intermediate frequency signal output from the tuner unit 17. Analog/digital switches 4 and 19 included in the demodulation unit 21 select an analog demodulator 5 during an analog broadcast reception, while the analog/digital switches 4 and 19 select a digital demodulator 6 during a digital broadcast reception.
The high frequency signal received by the antenna 1 passes through a first bandpass filter 8, a high frequency amplifier 9, and a second bandpass filter 10, is fed to a frequency converter circuit 18, and is frequency-converted. The intermediate frequency signal output from the tuner unit 17 passes through a surface acoustic wave filter 3 and the analog/digital switch 4, is fed to the analog demodulator 5 or to the digital demodulator 6, and is demodulating-processed.
During the analog broadcast reception, an output signal from the analog demodulator 5 is fed to the high frequency amplifier 9 of the tuner unit 17 as a high frequency gain control (RF AGC) signal to control gain of the high frequency amplifier 9. During the digital broadcast reception, an output signal from the digital demodulator 6 is fed to the high frequency amplifier 9 of the tuner unit 17 as a high frequency gain control (RF AGC) signal to control gain of the high frequency amplifier 9.
According to the analog/digital-compatible front-end module shown in FIG. 2, gain control (AGC) of the high frequency amplifier 9 is performed based on the intermediate frequency signal that has passed through the surface acoustic wave filter 3 with a narrow band, so that AGC control accurate to a desired wave is realized.
An analog/digital-compatible front-end module shown in FIG. 3 includes a tuner unit 2 for tuning-processing a high frequency signal received by an antenna 1, and a demodulation unit 20 for processing an intermediate frequency signal output from the tuner unit 2. An analog/digital switch 4 included in the demodulation unit 20 selects an analog demodulator 5 during an analog broadcast reception, while the analog/digital switch 4 selects a digital demodulator 6 during a digital broadcast reception.
The high frequency signal received by the antenna 1 passes through a first bandpass filter 8, a high frequency amplifier 9, and a second bandpass filter 10, is fed to a frequency converter circuit 11, and is frequency-converted. The intermediate frequency signal output from the tuner unit 2 passes through a surface acoustic wave filter 3 and the analog/digital switch 4, is fed to the analog demodulator 5 or to the digital demodulator 6, and is demodulating-processed.
During the analog broadcast reception or digital broadcast reception, an output signal (a high frequency gain control (RF AGC) signal) from an AGC signal detector circuit 15 connected to an output end of an intermediate frequency amplifier 14 included in the frequency converter circuit 11 is fed to the high frequency amplifier 9 to control gain of the high frequency amplifier 9.
According to the analog/digital-compatible front-end module shown in FIG. 3, gain control (AGC) of the high frequency amplifier 9 is performed based on the intermediate frequency signal that has passed through the bandpass filter 10 with a wide band, and has been frequency-converted, so that if a higher-level interference wave than a desired wave exists in a neighboring channel, the interference wave is detected and control for lowering the gain of the high frequency amplifier 9 is performed. Consequently, a level of a signal input to a mixer 13 or the intermediate frequency amplifier 14 included in the frequency converter circuit 11 becomes lower, and therefore is unlikely to exceed a saturation level for the mixer 13 or intermediate frequency amplifier 14. This leads to good distortion properties.
However, in the analog/digital-compatible front-end module shown in FIG. 2, if a higher-level interference wave than a desired wave exists in a neighboring channel, because the AGC is performed on a basis for a level of the desired wave, a level of the interference wave is likely to exceed a saturation level for a mixer 13 or an intermediate frequency amplifier 14. This causes a problem that distortion properties of the mixer 13 or intermediate frequency amplifier 14 become worse.
On the other hand, in the analog/digital-compatible front-end module shown in FIG. 3, if a higher-level interference wave than a desired wave exists in a neighboring channel, because the interference wave is detected and the AGC is performed, a signal level of the desired wave becomes lower, which causes a problem that S/N properties become worse.
Accordingly, an object of the present invention is to provide an analog/digital-compatible front-end module excellent in both S/N ratio, a decrease of which has been a problem in analog broadcast, and distortion properties, which have been a problem since digital broadcast started, whether broadcast to be received is analog broadcast or digital broadcast.